1. Field of the Invention
The present invention relates to inkjet printing of radiation curable inkjet inks that are etch resistant.
2. Description of the Related Art
Etching is the process of using a chemical, usually a strong acid or mordant, to cut into the unprotected parts of a metal surface in order to create a conductive pattern, e.g. a printed circuit board, or a design for decorative purposes.
Printed circuit boards are generally made by bonding a layer of copper over the entire substrate, applying a temporary mask and removing by etching unwanted copper leaving only the desired conductive copper pattern.
The temporary mask can be made by screen printing etch resistant ink onto the copper foil. The screen printing method by nature has low resolution and requires a relatively high viscosity for avoiding adverse phenomena such as bleeding and smudges.
Another method to provide a temporary mask is photoengraving. A photomask is usually prepared by laser-printing an image onto a transparent film using computer-aided manufacturing software. After exposing the photoresist coating on the copper foil through the photomask, a developer removes the non-exposed photoresist coating. Although high-resolution conductive patterns can be made, this method results in extra cost and chemical waste. Direct laser imaging techniques were developed which reduced the chemical waste.
U.S. Pat. No. 5,270,368 (VIDEOJET) discloses a UV curable, etch-resistant ink for inkjet printing circuit boards comprising a resin formulation having at least two acrylate components, one of which is an aromatic acrylate having a pendant carboxyl group and one of which is an acrylated epoxy monomer or dimer, a photoinitiator and an organic carrier. The preferred organic carrier of methanol and methyl ethyl ketone is employed in a range of 40% to 90% by weight of the ink composition. These volatile organic solvents lead to latency problems of inkjet print heads making reliable inkjet printing in an industrial environment process problematic. Reducing the amount of organic solvent leads to a too high ink viscosity, because some aromatic acrylate compounds traditionally used for preparing photoresist coatings have very high viscosity. For example, the bisphenol A ethoxylated diacrylate (Photomer™ 4028) used in all the examples of U.S. Pat. No. 5,270,368 (VIDEOJET) has a viscosity of 800 to 1200 mPa·s at 25° C. These aromatic acrylate compounds are essential for having a good balance in adhesion so that the printed ink layer is etch resistant yet easily removable after etching, especially since many different etching conditions and etchants are used in industry.
In another approach, polymerizable acidic compounds are included in the radiation curable ink to promote adhesion to the metal surface. For example US 2011024392 A (NISSHIN STEEL & TOKYO PRINTING INK MFG CO) discloses an etch resist inkjet ink having excellent adhesion to metallic plates by including a specific polymerizable phosphoric ester compound, a polyfunctional monomer having two or more ethylenic double-bond groups per molecule and having no phosphoric ester group and a monofunctional monomer having one ethylenic double-bond group per molecule and having neither phosphoric ester group nor carboxy group.
WO 2004106437 A1 (AVECIA) discloses a process for etching a metal or alloy surface which comprises applying an etchresistant ink by ink jet printing to selected areas of the metal or alloy, exposing the etch-resistant ink to actinic radiation and/or particle beam radiation to effect polymerisation, optionally thermally treating the etch-resistant ink and then removing the exposed metal or alloy by a chemical etching process wherein the etch-resistant ink is substantially solvent-free. All the disclosed etch-resistant inks include an acidic polymerizable compound.
WO 2004462260 A2 (MARKEM) discloses a radiation-curable hot melt ink composition comprising: a colorant; a polymerizable monomer; and a photoinitiating system comprising 0.5-1.5% by weight of an aromatic ketone photoinitiator, 2-10% by weight of an amine synergist, 3-8% by weight of an alpha-cleavage type photoinitiator, and 0.5-1.5% by weight of a photosensitizer.
However, the inclusion of acidic polymerizable compounds has some undesirable side effects such as increased viscosity and decreased ink stability and curing speed.
Hence, there remains a need for improved low viscous radiation curable inkjet inks suitable for reliable inkjet printing in an (industrial) etching process and applicable to a wide range of etchants and etching conditions.